Three-phase transformer

ABSTRACT

A three-phase transformer is presented comprising a magnetic circuit and three coil block. The magnetic circuit comprises two spaced-apart, parallel, plate-like elements; and three spaced-apart, parallel column-like elementary circuits. Each of the column-like elementary circuits carries the corresponding one of the three coil blocks, and serves for the corresponding one of the three phases. The column-like elementary circuits are substantially perpendicular to the plate-like elements, and are enclosed therebetween such as to form a spatial symmetrical structure about a central axis of the transformer.

FIELD OF THE INVENTION

This invention relates to a three-phase electrical transformer and amethod for manufacturing thereof.

BACKGROUND OF THE INVENTION

A transformer is a known electrical device widely used for transferringenergy of an alternating current in the primary winding to that in oneor more secondary windings. It typically contains two or more electricalcircuits comprising primary and secondary windings, each made of amulti-turn coil of electrical conductors with one or more magnetic corescoupling the coils by transferring a magnetic flux therebetween.

Presently known three-phase transformers usually utilize E+1 magneticcores in a flat structure. Such a transformer includes severalinterconnected magnetic cores located in one plane. U.S. Pat. Nos.4,893,400 and 5,398,402 disclose transformers having a magnetic coremade of an amorphous metal strip wound into a core over a mandrel, withone leg of the resulting core being subsequently cut off and withforming the metal into a rectangular shape. This transformer ismanufactured in the following manner. A piece of rectangular steel iswrapped around the outer periphery of the amorphous metal core. Theamorphous metal is then annealed, and the core is encapsulated in aresinous coating, except the cut leg. This allows the opening of the cutleg. The layers of amorphous alloy strips of the two edges are orientedso that the edges define top and bottom surfaces, each surface having adiscontinuity defining a distributed gap portion extending from the topsurface to the bottom surface. The coils are placed over two long legsand the cut leg is closed. The joint is then sealed.

According to U.S. '400, the sealing is made with glass cloth and anultraviolet-curable resin to provide the structure by the “fit and cure”method. This method is costly and labor-intensive. The transformershaving amorphous metal cores manufactured according to this methodcannot be repaired without causing damage to the core.

According to U.S. '402, the sealing is made with a porous material suchas woven cotton cloth or paper. The porous material is folded over thejoint and secured into position. An additional piece of porous materialis placed through the window of the core, wrapped around the core andsecured there. Electrical grade steel is disposed around the transformercore and is closed around the core joint and tack-welded. This structureallows the cut leg to be opened to permit replacement of a defectivecoil. The operation, however, is time-consuming and labor-intensive.

U.S. Pat. No. 5,441,783 discloses a technique of the kind specified,wherein a coating used to impregnate the core joint is a porous materialwith a viscosity greater than about 100,000 cps and a bonding materialwith a viscosity of at least about 100,000 cps. The porous materialcomprises strands of fiber, and the bonding material is thixotropicepoxy. Although the coated cores have good magnetic properties, theirmanufacture requires costly and complex operational steps. Moreover, themethod of repairing these cores is labor-intensive.

Another common disadvantage of the transformers manufactured accordingto the techniques disclosed in the above patents is that annealedamorphous metals become extremely brittle, and thus break undermechanical stress, for example, during the stage of closing the corejoint.

In the transformers of the above kind, a planar core structure is used.U.S. Pat. No. 4,639,705 discloses a transformer structure of anotherkind, having a spatial magnetic core system. This structure hasadvantages over the planar “E+1” structure, such as the reduced quantityof required magnetic materials (by about 20-30%), reduced volume of thetransformer, reduced core losses (by about 20-30%), and balancedcurrents in the three phases of the primary windings. However, tomanufacture a transformer in accordance with the technique disclosed inU.S. '705, complex production technology as well as a complex repairtechnology, are required.

SUMMARY OF THE INVENTION

It is accordingly a need in the art to facilitate the manufacture andmaintenance of a three-phase transformer, by providing a novelelectrical transformer structure and a method of its manufacturing.

It is a major feature of the present invention to provide such atransformer that has higher efficiency and smaller magnetic core, andthat uses lower quantities of materials per unit electrical power and/orhas better maintainability, as compared to those of the conventionaltransformers of this kind.

The main idea of the present invention consists of constructing athree-phase transformer having a spatial symmetrical structure of amagnetic circuit. The magnetic circuit comprises two spaced-apart,parallel plate-like elements, and three spaced-apart parallelcolumn-like elementary circuits, which are substantially perpendicularto the plates and are enclosed therebetween forming a mutuallysymmetrical structure.

There is thus provided according to one aspect of the present inventiona three-phase transformer comprising a magnetic circuit and three coilblocks, wherein the magnetic circuit comprises:

two spaced-apart, parallel, plate-like elements; and

three spaced-apart, parallel column-like elementary circuits, eachcolumn carrying the corresponding one of said three coil blocks andserving for the corresponding one of the three phases, wherein thecolumns are substantially perpendicular to the plate-like elements andare enclosed therebetween such as to form a spatial symmetricalstructure about a central axis of the transformer.

Preferably, each element of the magnetic circuit (i.e., plates andcolumns) is formed of an amorphous strip (e.g., ribbons of a softferromagnetic amorphous alloy) or a silicon steel strip. The plate-likeelement may be of a substantially triangular shape with rounded edges,or of a circular shape that simplifies the technological process of themanufacture of the plate-like element. The plate-like element may be atoroid.

Each of the column-like elementary circuits may be a toroid or severalaxially mounted toroids, each having a radial slot filled with aninsulating material. Alternatively, each of the elementary circuits maybe manufactured from a plurality of vertically aligned strips or ribbonpieces, in which case the cross section of the column is a polygon or acircle. The ribbon pieces are attached to each other, in such a mannerthat each ribbon piece is in a planar state and is oriented along thecolumn.

The elementary circuits are spaced from each other and from theplate-like elements by insulating spacers. All the spacers may be formedof plastic with filler of a magnetic powder with the concentration of20-50%.

Each of the toroids may be made of a set of amorphous strips havingdifferent widths. The alternation of the strips of different widthsextends along the vertical axis of the toroid, and the strips of theadjacent layers are displaced from each other along the vertical axis insuch a manner that the strips of one layer overlap the butts of thestrips of the adjacent layer.

The working surfaces of the toroidal plates can be formed with annularconcentric recesses, the butt-end surfaces of the vertical elements(columns) being formed with corresponding projections to be received bythe recesses. The contacting surfaces of the recesses and projectionsshould be coated with insulating materials.

The advantages of the present invention consist of the following. Theprovision of the plate-like elements of a triangular shape with roundedcorners allows for effectively transferring the magnetic flux betweenthe three column-like elementary circuits enclosed between the plates.The provision of the column-like elementary circuits formed by one ormore toroids produced by wounding the amorphous strips, enables toobtain a desired height of the column irrespectively of the limitedwidth of the strip. Moreover, the stacked structure of the column formedof several toroids provides good conductivity of the magnetic flux (lowreluctance) along the column, while presenting high impedance to eddycurrents. By forming the elementary circuit (column) with a radial slot,the eddy currents could be even more reduced. Actually, the introductionof the radial slot results in the induction of high voltage equivalentto that in one ribbon turn. Additionally, such a modular structure ofthe entire transformer simplifies its assembling and dismantling,thereby allowing the easy manufacture and maintenance of thetransformer. Thus, by appropriately selecting the dimensions of thetransformer's elements (e.g., the diameter of each column-like elementand each of the plate-like elements), the desired properties of thetransformer can be achieved.

According to another aspect of the present invention, there is provideda method for manufacturing a three-phase transformer, the methodcomprising the steps of:

(i) producing two substantially plate-like elements of a magneticcircuit of the transformer from materials having soft ferromagneticproperties;

(ii) producing three column-like elementary circuits of said magneticcircuit from materials having soft-ferromagnetic properties;

(iii) mounting a coil block on each of the column-like elementarycircuits to form the corresponding one of the three phases of thetransformer,

(iv) mounting the column-like elementary circuits between the plate-likeelements in a spaced-apart parallel relationship of the elementarycircuits, such as to form a spatial symmetrical structure about acentral axis of the transformer.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, a preferred embodiment will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIGS. 1 and 2 illustrate schematically exploded and assembled views of athree-phase transformer structure according to the invention;

FIG. 3 is a section taken along lines A—A in FIG. 2;

FIGS. 4 and 5 illustrate more specifically some constructional parts ofthe three-phase transformer of FIGS. 1-2, showing two possible examples,respectively, of assembling means for assembling the transformer;

FIG. 6 illustrates the principles of manufacturing the column-likeelementary circuit of the transformer of FIGS. 1-2, utilizing amorphousribbon strips of different widths;

FIG. 7 more specifically illustrates the structure of the elementarycircuit of the transformer of FIGS. 1-2, utilizing a plurality oftoroids;

FIG. 8 more specifically illustrates the structure of the end surfacesof the plate-like element and elementary circuit, showing the place ofjoint thereof;

FIG. 9 more specifically illustrates the structure of the elementarycircuit of the three-phase transformer, including longitudinallyoriented ribbon parts; and

FIGS. 10 and 11 illustrate two stages in a method of assembling thestructure of the elementary circuit of the transformer of FIGS. 1-2.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIGS. 1 and 2, the main components of a three-phasetransformer 10 constructed according to the present invention areillustrated. The transformer 10 comprises a magnetic circuit 12 formedby an upper plate-like element 14, a lower plate-like element 16, andthree parallel identical column-like elementary circuits, generally at18. The magnetic circuit 12 is arranged such that the plates 14 and 16are parallel to each other, and the columns 18 serve as supports betweenthe plates, thereby forming a cage-like structure spatially symmetricalabout a central axis CA. In the present example, each of the plates 14and 16 is a toroid, and is made of amorphous ribbons 22 wound about acentral hole 23 to form the planar toroid. Further provided are threecoil blocks 20, each for mounting on a corresponding one of the columns18. As shown in FIG. 2, each of the coil blocks 20 includes a primarywinding 20 a and a secondary winding 20 b. Thus, each phase of thetransformer 10 is formed by the column-like elementary circuit 18 withthe corresponding coil block 20 mounted thereon.

The transformer 10 has a modular structure, namely, the plates 14 and16, and the columns 18 can be easily assembled together anddisassembled, as will be described more specifically further below. Whenone of the plates 14 or 16 is removed, the coil blocks 20 can be removedas well, thereby enabling, for example, to repair the coil.

In the present example, each of the plates 14 and 16 has a generallytriangular shape with rounded sides and corners. After forming the plate14 of the desired shape and size, an excess-ribbon portion 22 a is cutoff. The amorphous ribbon 22 is made of an alloy having softferromagnetic properties, as required for the magnetic circuit of atransformer. Amorphous ribbon is known to have good ferromagneticproperties. The structure of the transformer 10 according to theinvention allows for beneficial use of these properties in a practicaltransformer structure.

Each of the columns 18 is also a toroid, or a plurality of toroidsstacked on top of each other—three toroids 18 a, 18 b and 18 c in thepresent example. This construction enables to achieve a desired heightof the column 18, notwithstanding the fact that the width of amorphousribbon is typically limited. Thus, the present invention allows forproducing a transformer with any desired height of the column-likeelementary circuit 18 by stacking toroids of limited height on top ofeach other.

As shown in FIG. 2, the entire structure is held together with threede-mountable bands 24 (only two of them being seen in the figure), eachhaving a screw (or spider) 26 to tighten the band. Structural members 28are provided, each located between the corresponding one of the bands 24and each of the plates 14 and 16. A base 30 supports the entirestructure. An inner, upper surface 16 a of the plate 16 is brought intocontact with lower surfaces of the columns 18 to transfer magneticfluxes therebetween, as will be described more specifically firer below.

FIG. 3 illustrates a section taken along line A—A of FIG. 2, showingmore specifically the lower plate 16 and the three columns 18 of themagnetic circuit 12. Each column 18 is formed with a central hole 32,and the columns 18 are arranged symmetrically about the central axis CA.As shown, the structural member 28 is located between the correspondingone of the bands 24 and the plate 16. The plate 16 preferably has aprotective coating 34 aimed at prolonging its life.

Turning back to FIGS. 1 and 2, the operation of the transformer 10consists of the following. As a current passes through each primarywinding 20 a of the coil block 20, a magnetic flux is generated andpropagates along the corresponding column 18 between the upper and lowerplates 14 and 16. Arrows 36, 38 and 40 show fluxes generated in thethree columns 18, respectively. The magnetic flux flowing through thecolumn. 18 generates an induced voltage in the secondary winding 20 b ofthe corresponding coil block 20. The device having this structure thusfunctions as a three-phase transformer.

Thus, the electric current, for example, with the working frequency of50 Hz, is supplied from a power source (not shown) to a terminal of coilof the primary winding 20 a, and, whilst passing through the coil turns,creates the basic magnetic flux 36. Let us now consider the moment ofpassing of the magnetic flux along one phase of the transformer.Assuming, for example, that at a given moment the flux 36 flows up.Then, the flux 36 is divided into two identical fluxes 42 and 44 in theplate 14. These fluxes 42 and 44 flow along two identical portions ofthe toroidal plate 14, and, then, flow down through the two other cores18. The flux 42 changes into flux 38, and the flux 44 changes into theflux 40 passing down through the columns 18. Then, the fluxes 38 and 40flow along two equal paths of the toroidal plate 16. Whilst passingalong the toroidal plate 16, the flux 38 changes into a flux 46, and theflux 40 changes into a flux 48. The fluxes 46 and 48 are transferredinto the column 18 forming the sum flux 36, which flows up. Thus, themagnetic flux loop is closed. The fluxes of the other phases of thetransformer flow in the similar way summing up the total magnetic flux.

As indicated above, the plates 14 and 16 could have a circular shape. Inthis case, the flux steams 42, 44, 46 and 48 will flow along circularpaths therein. In the example of FIGS. 1 and 2, each of the plates 14and 16 is shaped like an equilateral triangle with rounded sides andcorners. This results in a shorter path for the flux streams in theplates 14 and 16 between the columns 18, i.e., the shape of the fluxstreams is closer to a straight line. This enables to achieve a lowermagnetic reluctance, or better conductance of the magnetic flux. A moreefficient structure could be achieved by using a more raw material forthe magnetic core. To manufacture each of thee plate-like elements 14and 16, the amorphous ribbon 22 is secured to a mandrel of a triangularcross section, which is then rotated about its In axis. When the desiredsize of the plate 16 is achieved, the plate is fixed in that state usingeither impregnation or welding procedure, and the excess of ribbon 22 ais cut off. Due to the triangular cross-section of the mandrel, theplate 16 has a generally equilateral triangle shape with rounded cornersand sides.

Each winding in the coil block 20 is made of a copper wire. Each coilmay have a winding and a case insulation compatible with the workingvoltage and cooling system used. If air-cooling is used, a relativelythick insulation may be required. In case the transformer is immersed inoil, a thinner insulation may be used for the same voltage. Oil may beused for cooling as well as for insulation between the windings.

The cross-sectional area of the column 18 and the corresponding area onthe plates 14 and 16 are defined by the ferromagnetic property of theamorphous alloy these parts are made of, and by the transformer workingvoltage. The height of each column 18 and the distance between thecolumns is derived from the dimensions of the coil blocks 20, accordingto the cross-sectional area of the wires, the number of turns and therequired insulation. The dimensions of the plates 14 and 16 are such asto form a base for the whole cross-sectional area of all the columns 18,when the columns 18 are located at the required distance therebetween.This allows the passage of the magnetic flux from the columns 18 to theplates 14 and 16.

In the present example, each of the toroids 14, 16, 18 a, 18 b and 18 cis made of amorphous ribbon of about 20 mm in width and 25 μm inthickness. It should, however, be noted that the toroids 18 a, 18 b and18 c may be made from ribbons in the range of 10-100 mm wide, or asallowed by the ribbon manufacturing process.

FIG. 4 more specifically illustrates the column 18 of the magnetic core12 of the transformer and means for assembling the transformer. Thecolumn 18 is mounted between the upper and lower plates 14 and 16. Theprimary and secondary winding 20 a and 20 b of the coil block 20 aremounted on the column 18. The structure is held together with thede-mountable bands 24 which are tightened with the screws 26. Thestructural member 28 is located between the band 24 and each of theplates 14 and 16. The de-mountable bands 24, screws 26 and structuralmembers 28 constitute together the assembling means. It should be notedthat the type and size of the assembling means could depend on thedimensions and rated power of the transformer.

As the inner (upper) surface 16 a of the plate 16 comes in contact witha lower surface 50 of the columns 18 to transfer the magnetic fluxes inthe transformer, a narrow air gap 52 may be created therebetween. Thewidth of the gap 52 may, for example, be about 0.2 mm. This gap 52should preferably be filled with a magnetic paste, to improve theoverall ferromagnetic property of the magnetic loop, namely to decreasethe magnetic resistance. The magnetic paste may include an amorphouspowder with soft ferromagnetic properties, having particle size largerthan 20 μm, and a binding insulating material like transformer oil orepoxy resin. The concentration of the amorphous powder in the paste isusually between 50% and 90%. Any other suitable means can be used tominimize the gap 52 and its influence on the magnetic loop. An outer(lower) surface 16 b of the plate 16 may be formed with a protectivecoating.

Similarly, a narrow air gap 54 may be created between a surface 14 a ofthe element 14 and an upper surface 51 of the column 18. The gap 54should also be filled with a magnetic paste. An outer (upper) surface 14b of the plate 14 should preferably also be formed with a protectivecoating.

FIG. 5 illustrates one of the columns 18 of the magnetic circuit 12associated with a somewhat different assembling means, as compared tothat of the example of FIG. 4. To facilitate understanding, the samereference numbers are used for identifying those components, which areidentical in the examples of FIGS. 4 and 5. Here, the upper and lowerplates 14 and 16 and the column 18, are held,together by a threaded beamor screw 56. The structural members 28 that are attached to each of theplates 14 and 16 include means adapted for the thread and nut structure.

It is important to note that, when manufacturing transformers of variouspower, one comes into conflict caused by the absence of strips made ofamorphous materials with arbitrary width, and by the need for a magneticcircuit element having the height much larger than the strip's width.For example, the presently available strips have the width of 70 mm,while the required height of the toroid-like plate 14 (and 16) is 90 mm.To solve this problem, the toroid can be produced by winding the stripsof different widths, the total width of the strips being equal to theheight of the toroid. The strips in the adjacent layers of the toroidare displaced from each other such that the strips of one layer overlapa gap between the strips of the adjacent layer. Due to this windingtechnique, a toroid having desired dimensions can be obtained. In thistoroid, the even distribution of a magnetic flux is observed.

As illustrated in the example of FIG. 6, a winding of a 90 mm heighttoroid is carried out from strips 22 ^((a)) having the width of 70 mmand strips 22 ^((b)) having the width of 20 mm. The strips are locatedon four coils of a winding device (not shown), from which the strips 22^((a)) and 22 ^((b)) are sequentially supplied to the first layer, andthe strips 22 ^((b)) and 22 ^((a)) are sequentially supplied to thesecond layer. In this case, the toroid winding is carried out in twolayers simultaneously, each successive layer overlapping the gap betweenthe strips of the adjacent layer.

Reference is made to FIG. 7, more specifically illustrating thestructure of the column-like elementary circuit 18. In the presentexample, the column 18 is formed by the three toroids 18 a, 18 b and 18c. It should, however, be understood that the column 18 could be in theform of a single toroid. The column 18 can be fabricated similarly tothe plates 14 and 16, namely from several strips of different widths.All the toroids 18 a, 18 b and 18 c (or the single toroid) are formedwith the central hole 32. An outer cover 50 a of the toroid ispreferably made of an insulating material, for example, a glass-clothlaminate impregnated with an epoxy resin. The toroids 18 a, 18 b and 18c are made of amorphous ribbon, and preferably have a radial slot 70 todecrease losses and to prevent high voltages from being induced into thewindings of the toroids. Such a high voltage may cause breakdown of theinsulation between the adjacent layers of the toroid. The radial slot 70may, for example, be of 1 mm in width, or of any other appropriate widthfor a specific transformer design. The slot 70 may be made with acorundum disk (not shown) of 200 mm diameter and 0.5-1 mm thickness,using a cooling liquid and the toroid secured in a suitable fixture. Theslot 70 is preferably filled with an insulating material, for example aglass-cloth-base laminate. In the present example, cylinders 74 made ofan insulating material are inserted into the hole 32, so as to aligntogether the toroids 18 a-18 b and 18 b-18 c. The cylinders 74 may havea central hole, to allow the insertion of a threaded beam (not shown).

One of the parameters characterizing the operation of a transformer isthe idle current. This value depends on the characteristics of themagnetic materials used and the values of the air gaps 52 and 54 (FIG.4) between the separate parts of the magnetic circuit. The affect of theair gap can be reduced in the following manner:

The air gaps 52 and 54 are filled with a magnetic paste or with a spacermade of plastic having a filler of magneto-conductive powders, forexample, amorphous iron-based powders. The thickness of such a spacermay, for example, be 0.1-0.2 mm. The induction in the air gap isreduced, which can be achieved by increasing the cross sectional area ofthe air gap, through which the magnetic flux passes, by several times.

FIG. 8 illustrates one possible example of the implementation of thespacer. Here, annular, concentric recesses R are made in the workingsurfaces 16 a and 14 a of the toroid plates 14 and 16 (only the plate 16being shown in the figure). In the present example, the recesses R havethe thickness d of 3 mm and the depth h of 6 mm, the pitch b between theadjacent recesses R being 3 mm. Butt-end surfaces of the elementarycircuits 18 are formed with corresponding projections P to be receivedby the recesses R. The surfaces of the recesses R and projections Pshould be coated by an insulating material, such that an air gap G, forexample of 0.05 mm, is maintained between the side surface of eachprojection P and the side surface of the recess R.

FIG. 9 exemplifies the column-like elementary circuit 18 of thethree-phase transformer formed from the longitudinally orientedamorphous ribbon pieces 22. The ribbon pieces 22 may have the samewidth, e.g., 50 mm, or various width values. In the present example, the25 μm thickness ribbon pieces are used, although other thickness valuesare suitable as well. It should be noted that the cross-section of thecolumn 18 may have rectangular or polyhedral shape. The main advantageof this design is that the long column 18 may be obtained without theneed to stack parts thereof one on top another, as in the previouslydescribed examples. The elementary circuit 18, formed of thelongitudinally oriented ribbon pieces 22, is produced in the followingmanner:

An amorphous ribbon made of a ferromagnetic alloy is cut to pieces 22,each having the length equal to the height of the column 18 to beobtained. The cutting may be with the ±0.5 mm precision, and the burrsare filed off. The width of the ribbon pieces 22 is set in accordancewith the required cross-sectional dimensions of the column 18. Theribbon pieces 22 are stacked in an annealing fixture (not shown) to formthe column with the desired dimensions. The fixture includes a pressingmeans for pressing the pieces 22 together to achieve the desiredcoefficient of density, which is about 0.8-0.9. Annealing of thecomplete column 18 in its fixture at the temperature of about 350-550°C. is, preferably, performed in a furnace with controlled atmosphere,for a time period of less than one hour. The annealing procedure may beperformed with or without the application of an external magnetic fieldto the column. Should the application of the external magnetic field beused, such a field may be either longitudinal or transversal.Impregnation of the annealing package with an organic binding material,for example an epoxy resin, is performed in a vacuum chamber or in anultrasonic bath. The impregnation may be carried out with the pieces 22being in the annealing fixture. The column is placed in a thermostat andsintered at the temperature of about 80-105° C. Then, the column isremoved from the fixture, and the excess of the binding material isremoved from the planar surfaces at the top and bottom of the column.

To achieve better mechanical strength, the lateral surface of the columnis coated with a glass-cloth-base laminate band impregnated with epoxyresin that is wound about the column. After coating, the band issintered at the temperature of about 100-130° C. To provide sufficientlygood magnetic properties and allow for fitting the elements close toeach other (when assembling the column), the upper and lower surfaces ofthe column may be milled and polished to within 0.1 mm, with the totallength of the column being set to within a 0.1 mm tolerance. To preventstratification of the column during the machining process, it isnecessary to chuck the operated zone in a special fixture.

FIGS. 10 and 11 illustrate the main principles of assembling thetransformer 10. FIG. 10 shows the structure of the column 18 aftermounting the first coil of the coil block 20 (i.e., the secondarywinding 20 b) thereon. Spacers 80 made of an insulating material areused to mechanically attach the winding 20 b to the column 18, whilekeeping the parts electrically insulated from each other. Terminals 82of the winding 20 b are exposed to allow electrical connections thereto.During the formation of the structure, a specific distance d₁ is keptbetween the lower end of the winding 20 b and the lower end of thecolumn 18. The structure is symmetrical, having the same distance d₁ atthe upper end of the winding 20 b.

FIG. 11 shows the transformer 10 with both primary and secondarywindings 20 a and 20 b of the coil block 20 mounted thereon. The primarywinding 20 a is secured to the secondary winding 20 b by spacers 84. Thespacers 80 and 84 are made of an insulating material. Terminals 82 and86 are used to connect the secondary and primary winding 20 b and 20 a,respectively, to a power source and load (not shown).

Thus, the entire assembling procedure is performed in the followingmanner. The coil of the secondary winding 20 b is mounted on the column18 and secured thereon with the spacers 80. Then, the coil of theprimary winding 20 a is mounted on that of the secondary winding 20 band secured thereon with spacers 82, the coil 20 a being located in sucha manner as to keep a predefined distance d₂ from each of the ends ofthe column 18. The coils of the other two phases are mounted on thecorresponding columns 18 in a similar manner.

Turning back to FIG. 2, the plate 16 is set in a horizontal positionwith the working surface 16 a pointing upwards. This working surface isthe planar surface of the toroid 16 that was previously cleaned from theexcess of the impregnating material and, optionally, polished.

Thereafter, a layer of the magnetic paste, having the thickness about0.2 mm, is deposited on the plate 16 in the areas where the columns 18are to be mounted. The three columns 18 with coil blocks thereon aremounted on the plate 16 symmetrically about the central axis CA. Then,another layer of the magnetic paste, having the thickness about 0.2 mm,is deposited onto the upper surfaces of the columns 18, and the upperplate 14 is mounted on the three columns 18 to complete the structure.

As described above, the elements 14, 16 and 18 of the magnetic circuit12 are secured to each other using three de-mountable bands 24 with thescrews 26 to tighten each band. The structural members 28 made of aninsulating material are located between the bands 24 and the plates 14and 16. The screws 26 are rotated so as to tighten the bands, thussecuring the transformer parts together. Rotating the screws 26 in theopposite direction can easily dismantle the transformer. The bands 24become loose and allow the removal of the columns 18 and the plates 14and 16. Each coil can be then removed from its column, if desired.

The above technique allows for multiple cycles of dismantling/assemblingthe transformer, without causing any damage to the constructional partsof the transformer. This may facilitate the repair of the transformer,and may save work and materials needed therefor.

Various parts of the transformer may be separately and concurrentlyproduced, and then assembled together in the final step. The entiremethod of manufacturing the transformer consists of the following.

Initially, the amorphous ribbons 22 are produced from an alloy havingsoft ferromagnetic properties, as will be described more specificallyfurther below. Then, the elements (e.g., toroids) 14, 16, 18 a-18 c ofthe magnetic circuit 12 are produced. Each column-like elementarycircuit 18 may comprise one or several toroids, according to therequired height of the column 18 and the width of each toroid. In thecase that the column 18 includes several toroids, each of the columns isassembled from these toroids. The coil block 20 is assembled (in theabove-described manner), each including the primary and secondarywindings 20 a and 20 b. Alternatively, each winding may be separatelyproduced and assembled as a separate unit. Then, the impregnation and/orcoating of the elements and/or at windings are carried out. To assemblethe transformer from the so-produced elements, the columns 18 areinserted into the corresponding coil blocks 20, the coils are secured inplace, the columns 18 are mounted at the corners of the plate 16, andthe plate 14 is mounted on the columns 18. All the constructional parts14, 16 and 18 are secured together using screws, tension bands orsimilar mechanical means.

The preparation of the amorphous ribbon toroids will now be described.At present, to obtain sufficiently good magnetic properties, the as-castamorphous ribbons are annealed at a temperature of about 350-550° C. Thedisadvantage of this known method is that the amorphous ribbons becomeextremely brittle after annealing, usually breaking under mechanicalstress or during winding of a toroid. To overcome this deficiency, thepresent invention utilizes the following preparation scheme:

Coating an as-cast amorphous alloy ribbon with an insulating layer. Thethickness of the two-sided insulation needs to be no more than about 5μm. It should, however, be noted that for a low-voltage transformer,this stage may be omitted;

Winding of a toroid (like the toroids 14, 16, 18 a-18 c) from theas-cast ribbon. The winding procedure is carried out as described above,by using the steel mandrel. For the parts 14 and 16, the cross-sectionalarea of the mandrel 60 is triangular, and the mandrel thickness ispreferably substantially equal to the width of the ribbon to be wound.The mandrel 60 should have rounded corners to prevent cracks in theamorphous ribbon, for example corners with the radius about 10 mm. Forthe toroids 18 a, 18 b and 18 c, a cylindrically shaped mandrel is used.The mandrel's diameter depends on the dimensions of the toroids to bemanufactured, and may be in the range of about 10-30 mm. The mechanicaltension in the ribbon is set according to the required winding densitycoefficient, which usually is about 0.8-0.9. To force the layers of thetoroid to be laid exactly on top each other, the mandrel may have cheeksor delimiters mounted thereon. Using this scheme, the variation intoroid's width may be limited to a small value, for example about ±0.2mm.

The last layer of the toroid is secured to the adjacent layer to preventthe toroid from unfolding. This may be achieved, for example, by usingresistance welding.

Annealing of the complete toroid at a temperature of about 350-550° C.,preferably in a furnace with controlled atmosphere, for a desired timeperiod defined by the type of metal. The toroid may be annealed with themandrel still inserted therein. Annealing may be performed with orwithout the application of an external magnetic field (longitudinal ortransverse) to the toroid.

Impregnation of the toroid with an organic binding material, forexample, an epoxy resin in a vacuum chamber or in an ultrasonic bath.After the impregnation, the toroid is placed in temperature-controlledenvironment. The impregnation may be performed with the mandrel still inthe toroid.

The mandrel is removed from the toroid. The excess of an impregnationmaterial is removed from the planar surfaces of the toroid, or at leastthe surface of one of the elements 14 and 16. The working surfaces(areas used to transfer the magnetic flux) may be polished to obtainplanar surfaces for good flux transfer and low magnetic resistance; Theends of the toroid may be made parallel to within 0.2 mm. It should benoted, that the polishing procedure can be performed prior to the stepof annealing, while the toroid already has a fixed shape, and theamorphous ribbon is not yet brittle and is thus more workable.

As described above with reference to FIG. 7, for the toroids 18 a, 18 band 18 c, the radial slot 70 may be cut in the toroid. The Slot 70 maybe made with a corundum disk (not shown) of a 200 mm diameter and 0.5-1mm thickness, for example, by using a cooling liquid and with the toroidsecured in a suitable fixture. The slot 70 is preferably filled with aninsulating material, for example, a glass-cloth-base laminate.

To achieve better mechanical strength, the lateral circular area of thetoroid is coated with a glass-cloth-base laminate band that is woundabout the toroid. After the coating procedure, the band is sintered atthe temperature of about 100-130° C.

It should be noted that all the magnetic circuits in the transformerhaving the above construction could be manufactured not only fromamorphous materials, but also from silicone steel. Although this leadsto the increased losses in the magnetic circuit, it enables to simplifythe technological process, owing to the fact that a strip of therequired width can be selected for manufacturing the toroid. Therefore,the above construction utilizing silicone steel can be used in theapplications having reduced requirements to the effectiveness of thetransformer.

The technological process of the manufacture of the magnetic circuitfrom silicone steel consists of the following:

The toroidal plate (14 and 16) is wound from the strip produced fromsilicone steel having, for example, the width of 0.3 mm and aninsulating coating of 3-10 μm thickness. In this case, the coefficientof the winding density lies in the range of 0.8-0.96. The width of thestrip corresponds to the height of the toroidal plate.

After the winding procedure, the plate is impregnated by an insulatingvarnish, e.g., vacuum or ultrasound impregnation. The varnish solidifiesat the temperature of 80-105° C.

A bandage made of a glass-strip is wound along the perimeter of theplate, and then impregnated by epoxide varnish with furtherthermo-treatment at the temperature of 80-105° C.

The working surface of the plate is treated, e.g., milled, for obtaininga plane with the unevenness value not exceeding 10 μm.

The column like elementary circuits 18 can be manufactured similar tothe toroidal plates 14 and 16, or, alternatively, similar to a linearmagnetic circuit (FIG. 9). When using the toroid manufacturingtechnology, the width of the strip is selected to be larger than theheight of the column on the allowance value of mechanical treatment,e.g., 2 mm. The mechanical treatment of both butt-ends of the column 18,in distinction to that of the plate 14 and 16, is performed with theunevenness value not exceeding 10 μm and the unparallelism of thebutt-ends not exceeding 20 μm. Moreover, the longitudinal slot 70 (e.g.,of 1 mm in thickness) is made, and a plate (not shown) made of aninsulating material, for example glass-textolite (resin-dipped fabriclaminate), is inserted into the slot 70. A bandage made of a glass-stripis wound on the outer surface of the column, and then impregnated byepoxide varnish with further thermo-treatment at the temperature of80-105° C.

When manufacturing the column 18 in accordance with the constructionshown in FIG. 9, the silicone steel strips are set in the form ofpackets of different widths forming a polygon or a circle in the crosssection. The length of the strip is selected to be larger than theheight of the magnetic circuit on the allowance value of mechanicaltreatment, e.g., 2 mm. The assembled columns are impregnated by aninsulating varnish, e.g., epoxide, and undergo thermo-treatment underthe temperature of 80-105° C. A bandage of a glass-strip wound on thecolumn along its perimeter is impregnated by epoxide varnish and driedat the temperature of 80-105° C. Thereafter, mechanical treatment of thebutt-ends is performed with the unevenness value not exceeding 10 μm andunparallelism of the butt-ends not exceeding 20 μm.

Following are the calculation results corresponding to the transformerof 400 kVA power having the above design of assembling the separateparts of the magnetic circuit 12 to each other:

the cross sectional area of the column-like elementary circuit,S_(core)=293 cm²;

the surface area of the projections having the height of 6 mm in at thebutt-end of the column, S¹=469 cm²;

the butt-end surface area of the projections, S²=150 cm²;

the total area on the projections, along which the magnetic flux passes,S_(Σ)=619 cm².

In this case, for magnetic induction, we have:$B_{\delta} = \frac{B_{m} \cdot S_{core}}{S_{\Sigma}}$

wherein B_(m) is the induction in the column. When B_(m)=1.3(T),B_(δ)=(1.3×293)/619=0.61(T), which results in the reduction of idlecurrent by two. When selecting the depth of the recess equal to 12 mm,the idle current reduces by 4.

Mathematical analysis of a transformer made according to the presentinvention was performed, and results were compared to those for aconventional transformer having an “E+1” magnetic circuit structure. Theevaluation relates to the transformer having rated power values of 10kVA, 25 kVA, 100 kVA and 630 kVA. The analysis includes computation ofthe core and winding electrical losses and weight. All calculations wereperformed for a fixed, predefined value of overall efficiency.Calculation results are presented below in Tables 1 to 5.

Following are the parameters, which are common to all the tables 1-5:

f=50 Hz, wherein f is the working frequency;

three phase transformer;

Following are the variables in the tables 1-5:

P_(W), wherein W is the winding loss;

magnetic circuit loss P_(Fe) (W);

winding weight G_(W) (kg);

magnetic circuit weight G_(Fe) (kg);

total transformer weight G_(tr) (kg);

efficiency η (%);

transformer height B_(tr) (mm);

transformer length L_(tr) (mm);

transformer width B_(tr) (mm);

transformer volume V_(tr) (m³);

output power P₂ (kVA);

primary voltage U₁ (V);

secondary voltage U₂ (V)

TABLE 1 P₂ = 10 kVA; U₂ = 220 V; U₁ = 380 V Type of transformerParameters AMT, dry - Israel TSZM-10/0.4 Core design Toroid E + 1 typeCore material Amorphous metal Silicon steel P_(W) (W) 330 256 P_(Fe) (W)12 78 G_(W) (KG) 26 59 G_(Fe) (kG) 58 40 G_(tr) (KG) 85 99 η (%) 96.796.7 H_(tr) (mm) 214 465 L_(tr) (mm) 349 600 B_(tr) (mm) 349 335 V_(tr)(m³) 0.026 0.093

TABLE 2 P₂ = 25 kVA; U₂ = 220 V; U₁ = 380 V Type of transformerParameters AMT, dry - Israel TSZM-25/0.4 Core design Toroid E + 1 typeCore material Amorphous metal Silicon steel P_(W) (W) 697 558 P_(Fe) (W)19.3 157 G_(W) (KG) 64.5 133 G_(Fe) (kG) 95.5 77 G_(tr) (KG) 160 200 η(%) 97.2 97.2 H_(tr) (mm) 242 555 L_(tr) (mm) 441 706 B_(tr) (mm) 441463 V_(tr) (m³) 0.047 0.18

TABLE 3 P₂ = 100 kVA; U₂ = 380 V; U₁ = 22.5 kV Type of transformerParameters AMT dry - Israel Siblok, dry Core design Toroid E + 1 typeCore material Amorphous metal Silicon steel P_(W) (W) 2024 1700 P_(Fe)(W) 48 440 G_(W) (KG) 132 160 G_(Fe) (kG) 238 405 G_(tr) (KG) 371 565 η(%) 97.9 97.9 H_(tr) (mm) 706 1180 L_(tr) (mm) 1270 1300 B_(tr) (mm)1270 925 V_(tr) (m³) 1.13 1.41

TABLE 4 P₂ = 630 kVA; U₂ = 380 V; U₁ = 22.5 kV Type of transformerParameters AMT dry - Israel Siblok, dry Core design Toroid E + 1 typeCore material Amorphous metal Silicon steel P_(W) (W) 7071 5600 P_(Fe)(W) 136 1600 G_(W) (KG) 650 570 G_(Fe) (kG) 683 1740 G_(tr) (KG) 13332310 η (%) 98.87 98.87 H_(tr) (mm) 866 1850 L_(tr) (mm) 766 1820 B_(tr)(mm) 766 1186 V_(tr) (m³) 0.51 4.05

TABLE 5 P₂ = 630 kVA; U₂ = 380 V; U₁ = 22.5 kV Type of transformerParameters AMT; dry - Israel Allied Signal, Oil, USA Core design ToroidE + 1 type Core material Amorphous metal Amorphous metal P_(W) (W) 58805835 P_(Fe) (W) 148 186 G_(W) (KG) 537 487 G_(Fe) (kG) 739 932 G_(tr)(KG) 1276 1419 η (%) 99.05 99.05 Oil − + Tank − +

The computations for the transformers having various power ratings andvoltage levels indicate the advantageous features of the transformerconstructed according to the present invention, including among othersthe following features:

decrease of total weight by about 14% to 43%;

decrease in cost by about 3%-22%;

decrease in transformer volume by about 20% to 87%.

An experimental transformer manufactured according to the presentinvention has the following parameters:

P ₂=1 kVA; U ₁=380 V; U ₂=220 V; f=50 Hz; η=92.66%; G _(tr)=16.4 kg

It was found that this transformer has good maintainability, and theabove-described modular structure thereof enables its easy dismantlingand reassembling, while the conventional transformer of the kindspecified has the following characteristics: η=91% and G_(tr)=20 kg. Itis thus evident that the structure according to the invention enables toachieve the 18% decrease in the transformer weight at higher efficiency.

Those skilled in the art will readily appreciate that variousmodifications and changes can be applied to the preferred embodiments ofthe invention as hereinbefore exemplified without departing from itsscope defined in and by the appended claims.

What is claimed is:
 1. A method for manufacturing a three-phasetransformer, the method comprising: (i) producing two substantiallyplate-like elements of a magnetic circuit of the transformer fromamorphous strips, wherein each of the plate-like elements being producedas a planar toroid of a desired shape by winding at least one amorphousstrip about a central hole; (ii) annealing each of the planar toroids;(iii) impregnating each of the annealed planar toroids by a bindingmaterial; (iv) producing three column-like elementary circuits of saidmagnetic circuit from amorphous strips, wherein each of the column-likeelementary circuits is produced as a toroid of a desired height bywinding at least one amorphous strip about a central axis; (v) annealingeach of the column-like toroids; (vi) impregnating each of the annealedcolumn-like toroids by a binding material; (vii) forming each of theimpregnated column-like toroids with a radial slot extending along theheight of the column-like toroid and filled with an insulating material;(viii) mounting a coil block on each of the column-like toroids with theslot to form the corresponding one of the three phases of thetransformer; (ix) attaching opposite butt-end surfaces of each of thecolumn-like toroids to the plate-like elements, respectively andarranging the column-like toroids in a spaced-apart parallelrelationship, such as to form the magnetic circuit of the transformer asa spatial symmetrical structure about a central axis of the transformerpresenting the closed magnetic circuit for magnetic flux propagationtherethrough, spacers between the elements of the magnetic circuit ofthe transformer being filled with a material containing a magneticpowder.
 2. The method according to claim 1, wherein in step (i) thestrip is secured to a mandrel having a triangular cross-section androtatable about its central axis, and, upon obtaining a desired size ofthe plate-like element by rotating the mandrel, the element is fixed inthe obtained state and excess of the strip is cut off.
 3. The methodaccording to claim 1, wherein the fixing of the planar toroids and ofthe column-like toroids also includes welding of the ends of theamorphous strips.
 4. The method according to claim 1, wherein in step(i) several amorphous strips are wound having different widths, thetotal width of the strips being equal to the desired height of theplate-like element.
 5. The method according to claim 4, wherein thestrips in the adjacent layers of the plate-like element are displacedfrom each other such that the strips of one layer overlap a gap betweenthe strips of the adjacent layer.
 6. The method according to claim 1,wherein in step (iv) each of the column-like toroids is produced bymounting several toroidal elements on top of each other.
 7. The methodaccording to claim 1, wherein in step (iv) said amorphous strips havedifferent widths, the total width of the strips being equal to thedesired height of the toroid.
 8. The method according to claim 7,wherein the strips in the adjacent layers of the toroid are displacedfrom each other such that the strips of one layer overlap a gap betweenthe strips of the adjacent layer.
 9. A three-phase transformercomprising a magnetic circuit and three coil blocks, the transformerbeing manufactured according to the method of claim
 1. 10. The methodaccording to claim 1, wherein the annealing of each of the toroids iscarried out in a magnetic field.
 11. The method according to claim 1,wherein temperature of the annealing process is up to about 550° C. 12.The method according to claim 1, wherein in step (iii) said annealedplanar toroids are impregnated by a first binding material, and in step(vi) said annealed column-like toroids are impregnated by a secondbinding material.